Is there any point in owning a license for IntelliJ and RubyMine/PyCharm? I thought that IntelliJ Ultimate had all the features of RubyMine and PyCharm but I haven't used them so I could be mistaken.
You are correct, Ruby and Python plugins add most of RubyMine/PyCharm features to IntelliJ IDEA Ultimate. You may want to have separate IDEs if you need more lightweight environment. RubyMine & PyCharm are in general updated more frequently too.
RubyMine and PyCharm feel more light weight, meaning quicker and less resource use.